1. Field of the Invention
The present invention relates to computer-based tools for manipulating digital images. More specifically, the present invention relates to an improved Poisson image-editing technique, which matches both pixel values and texture contrast while seamlessly reconstructing a region of an image.
2. Related Art
In recent years, a number of effective techniques have been developed to digitally remove scratches, wires and other objects from images and video. For example, U.S. Pat. No. 6,587,592, entitled “Generating Replacement Data Values for an Image Region,” describes a system that implements a Poisson image-editing technique which “heals” a defective region of an image using data from a texture area from the same image or from a different image.
In Poisson editing, the defective pixels are replaced with new pixels described by a function ƒ(x, y), which is a solution of the Poisson equationΔƒ(x, y)=Δg(x, y)   (1)with a Dirichlet boundary condition which constrains the new ƒ(x, y) to match the original image at the boundary. In equation (1), g(x, y) is the texture that is “seamlessly-cloned” into the reconstructed area, and
                    Δ        =                                            ∂              2                                      ∂                              x                2                                              +                                                    ∂                2                                            ∂                                  y                  2                                                      .                                              (        2        )            
This technique effectively clones texture, while changing the color/shading of the replacement pixels to seamlessly match surrounding pixels at every boundary point.
While this method works very well in many cases, it sometimes encounters problems. Consider the example illustrated in FIGS. 1A, 1B and 1C. FIG. 1C illustrates how the scratch in FIG. 1A is ultimately removed by Poisson cloning of pebbles from the illuminated area into the shadow area. This technique correctly matches pixel values at the boundary of the patch, but the cloned pebbles are still easy to spot because there is too much variation and too high contrast in the reconstructed area of the image. Note that this problem is inherent in the nature of the Poisson equation (1), which transfers variations of g directly, without modification.
What is needed is a technique for digitally removing scratches, wires and other objects from images, without the above-described problems in matching variation and contrast.